Guitar Amp Repair - Mods - Jensen Speakers



Cap Job

Over time (20 to 25 years in many cases), electrolytic filter and bypass capacitors (caps) begin to go bad. This causes various problems in your amp including poor sound quality, weak output,  loss or reverb, intermittent tremolo, hum or “ghost” notes.  Replacing these caps often makes a big difference in the overall performance of your amp (if they are starting to fail).  In addition, bad filter caps sometimes leak current and can destroy your expensive power transformer.  See photos of bad caps below.  Electrolytic caps contain a semi-liquid paste that tends to dry and crystallize over the years.  When that happens, the capacitor no longer functions properly.  This type of capacitor is used in the main power supply, bias supply and in the cathode portion of the audio signal path.  When any of the electrolytic caps fail, there is a strong possibility that the others will soon follow since they are the same age and have been in the same environment.  When one of them is being replaced, it's an excellent time to have to others done to prevent the amp from having to go back into the shop soon for each successive cap failure.  Many customers prefer to have them replaced preventively when the amp reaches about 30 years of age even if they haven't yet failed.  I see a lot of bad electrolytic caps in amp built before 1980.  Recently, I've been seeing more 80s amps with bad ones as well.  It's rare to see them bad in amps built after 1990 except for a few bad batches of caps by Ruby and Illinois Capacitor and a couple of others.  I've also seen a few Sprague Atoms go bad after less than 10 years.  I've never seen a bad cap made by F & T, Xicon or Nichicon unless it was damaged by the failure of another component. 

Also, amps built prior to about 1960 usually used wax or oil type coupling caps.  Almost 100% of these caps are failing now.  If they are not replaced, the amp will sound bad and current will leak from the caps causing tubes to run too hot and fail as well as other symptoms.


Consignment Items (Amps and More)




The Sprague Atom caps have been the industry standard for many years but prices for "Atoms" have been going way up.  As of this time, a Cap Job using "Atoms" for most medium to large vintage Fender amps will cost around $145 to $170 depending on the model.  This includes all filters, bypass caps (also Sprague Atom) and selected coupling caps.  Other brands of amps vary widely from as little as $65 to over $200. 

We're now finding that other brands of caps often exceed the quality of "Atoms" I have switched  over to some of these such as the F & T brand from Germany, Xicon high-temp caps and others.  This means we can offer higher quality parts at lower prices.  For example, the same $155 Cap Job Plus that we can do on a Super Reverb using all "Atoms" can be done with F&T filters and Xicon high temp bypass caps for $135.  Both the F&T and the Xicon high temp caps are of very high quality.  We will still offer Sprague Atoms by request but we usually don't have them in stock.

These caps were removed from a 1962 Blond Tremolux and are typical of what happens to this type of cap after 40 years.  The white powdery substance oozing from the caps is the electrolyte that has begun to crystallize.   Also notice the oily satins on the cardboard.  When the electrolyte crystallizes, it expands.  Most electrolytic caps have vent holes to allow the escape of the expanding chemical to keep the cap from exploding when this happens.   That's what is happening to these caps.  Using an amp with caps in this condition is asking for trouble!

Here is a close up of 2 of the caps.  Notice the small dimple starting to form on the left cap.